U.S. Pat. No. 3,246,641 Goehring granted Apr. 19, 1966 and assigned to the assignee of the present invention discloses a combination engine protective device operable to stop operation of an internal combustion engine upon the happening of certain abnormal conditions such as, for example, excessive crankcase or air box pressure or inadequate water pressure in the engine cooling system. The device is operative upon occurrence of any of these abnormal operating conditions to open a valve so as to drain oil from a pressure line connected with the engine governor. The pressure drop in the oil line in turn actuates means in the engine governor to stop operation of the engine.
The water-air box pressure portion of the protective device, as described in column 4 of the Goehring patent, comprises a latching member (sleeve 62) engaged by a pair of diaphragms, one of which is responsive to pressure in the engine cooling system to urge the member toward its "latch" position and the other of which is responsive to pressure in the engine air box to urge the member toward its "release" position. In normal engine operation, the pressures in both the cooling system and the air box vary as functions of engine speed and so tend to offset one another. If, however, the engine water pressure is reduced or the air box pressure is increased an abnormal amount, the latching member is moved to its "release" position, tripping the protection device and stopping the engine.
Engine protective devices of the type disclosed in the above-cited patent have been satisfactorily used in many engine applications, particularly on diesel engines applied to railroad locomotives. In the past, the cooling system of such engines has generally been provided with a vent to atmosphere in order to maintain a relatively constant pressure level in certain portions of the cooling system, such as at the pump inlet. Currently, however, it has become common practice to close the cooling system vent by applying a pressure cap and providing the system with a pressure relief valve so that the cooling system may be pressurized up to a predetermined level. This permits operation at higher coolant temperatures without causing cavitation of coolant in the pump which sometimes causes inadequate coolant flow through the engine.
The protective device of the above-cited patent has been satisfactorily operated with both types of cooling systems. However, it has been found that, with closed cooling systems, the system may be pressurized to such an extent that the pump outlet pressure sensed by the protective device will remain high enough to keep the device in its latched position. In such an instance, the protective device will not shut down the engine, even though cooling water flow through the engine becomes inadequate for its continued operation at normal temperatures.